I'm not a parent so it still fascinates me how others strike a balance between letting them learn deep lessons by making their own mistakes and how much to try the "stitch, in time" approach.
Dishing out earnest advice in a way which won't be received as an instruction (or worse, an order), something to "rebel against" must be hard. Not giving it and having them later pleading "why didn't you warn me?" must be even worse.
Has it sunk in with him just how much she is disliked by others in the family?